云数据库数组的相关方法不能用?

const db = cloud.database()
const $ = db.command.aggregate
const _ = db.command   

 let res = await db.collection('plan')
      .aggregate()
      .project({
        first: $.arraElemAt(['$content', ]),
      })
      .end()

报错:TypeError: $.arraElemAt is not a function